This invention relates to an improvement in a monolithic device commonly known as "Darlington transistor" which is a device having Darlington connection of transistors formed on one monolithic substrate with their collectors connected in common.
As is well known, the Darlington circuit is the circuit capable of obtaining a high current amplification coefficient and is used in a circuit which requires a high current amplification coefficient.
Recently, so-called Darlington transistors which are composite semiconductor devices each having Darlington connected transistors formed on one monolithic substrate has been proposed, for instance by U.S. Pat. No. 3,836,995 for Carl Franklin Wheatley Jr. et al.
FIG. 1 is a circuit diagram of one example of a two-stage Darlington transistor circuit wherein numeral 1 is called base terminal, numeral 2 is called collector terminal and numeral 3 is called emitter terminal, respectively of the composite device. The device performs current amplification with great amplification coefficient when the abovementioned three terminals 1, 2 and 3 are connected regarding them as the base, the collector and the emitter of a transistor, respectively.
The Darlington transistor comprising a first transistor of smaller output, i.e., a driving transistor Tr1 and a second transistor, i.e., an output transistor Tr2 of larger output, connected by their collectors in common and by the emitter of the first transistor Tr1 to the base of the second transistor Tr2. Between the base and the emitter of the first transistor Tr1 a first resistance R1 is connected, and between the base and the emitter of the second transistor Tr2 a second resistance R2 is connected. The first resistance R1 is generally selected to be between 100.OMEGA. and 10K.OMEGA., the second resistance R2 is generally selected to be between 30.OMEGA. and 1K.OMEGA., and the value of the first resistance R1 should be selected to be larger than that of the second resistance R2.
FIG. 2(a) and FIG. 2(b) are plan view and sectional side view by B--B plane of FIG. 2(a) of a conventional Darlington transistor of mesa type NPN type, respectively, wherein the right part forms the first transistor Tr1 and the left part forms the second transistor Tr2. On an N-type silicon substrate 110 to become the common collector region 11-11', P-type base regions 17 and 17' are continuously formed by known epitaxial growth method. In the base regions 17 and 17', emitter regions 13 and 13' for the first and the second transistors Tr1 and Tr2 are formed, respectively, by known diffusion. In the second transistor Tr2 a small window part 14 is excluded from the diffusion to form the emitter region 13' so as to be retained as a part of the base region 17'. The base and emitter electrodes of both transistors Tr1 and Tr2 are formed by vapor deposition of aluminum.
The emitter electrode 12 of the second transistor Tr2 is formed to have a pattern smaller than that of the emitter region 13' and over the non-diffusion window part 14. An end part 13" of the emitter region 13' forms an obstruction region. Through the non-diffusion window part 14 and a narrowed path under the obstruction region 13" of length l2, the emitter electrode 12 and the base electrode 15" of the second transistor Tr2 are connected to each other. The narrowed path in the base region 17' and under an obstruction region 13" forms the second resistance R2, the value of which is determined by the area of the non-diffusion part 14, the length l2 of the obstruction region 13", with W2 of the obstruction region 13" and heights h2 of the narrowed path in the base region.
In the first transistor Tr1, the base electrode 16 is formed in an opening in the emitter electrode 15, and the emitter electrode 15 is formed in a manner that at least an extended part 15' of it contacts the base region 17' of the second transistor Tr2. The part 15' functions partly as the emitter electrode of the first transistor Tr1 and at the sametime as the base electrode of the second transistor Tr2. The emitter region 13 of the first transistor Tr1 is formed to have a pattern having an obstruction part 131 of length l1 between the base region 17 of the first transistor Tr1 and the base region 17' of the second transistor Tr2. A second narrowed path is formed between the base electrode 16 and the extended part of the emitter electrode 15' of the first transistor Tr1. The second narrowed path forms the first resistance R1, the value of which is determined by the heights h1, the length l1 and the width W1 of the narrowed path.
As is described in the above, the values of the first resistance R1 and the second resistance R2 are dependent on the lengths l1 and l2 of the obstruction regions 131 and 13" and the heights h1 and h2 of the narrowed paths of the first and the second transistors, respectively.
In order to obtain a high current amplification coefficient, the values of the resistances R1 and R2 should be as large as possible. However, for excessively large resistance values of R1 and R2, the stability of the operation is decreased. In the structure of FIG. 2, it is easy to obtain the resistance R2 of 30.OMEGA. to 500.OMEGA., but to obtain a high value of the resistance R1 which should have sufficiently large resistance value in comparison with that of R2 is not easy. If the length l1 of the obstruction region is made large in order to attain a large resistance value of R1, then the total size of the Darlington transistor becomes large, or in case the total size of the transistor is limited the size of either or both transistors Tr1 or Tr2 must be made small. If the height h1 of the narrowed path is made small in order to attain a large resistance value of R1, since the obstruction region 131 and the emitter regions 13 and 13' of the transistors Tr1 and Tr2 are formed in the same diffusing step, the gaps between the collector-base junction 19 and the base emitter junctions 18, 18' become small. Accordingly the characteristics of both transistors changes, for instance to lower energy breakdown levels. Therefore, decreasing of the height h1 should not be used for obtaining a high resistance vlaue of R1. Thus, in the conventional Darlington transistors, the necessary high resistance values of R1 have been obtained by using a considerably large size wafer.
There has been the following second problem in the Darlington transistor. Namely, when the Darlington transistor is used for switching use, a transient phenomenon that a reverse high electric power is consumed between the base and emitter electrodes likely to take place, the high power fades out during passing through the transistor. The shorter the fading out time is, the faster the switching speed of the switching device is. Therefore it is desirable to make the fading out time as short as possible. However, in the Darlington transistor of FIG. 1 formed in one monolithic circuit, it is difficult to make the fading out time short because of the following reason. As shown in FIG. 1, in the Darlington circuit, two junctions, namely the base-emitter junction of the first transistor Tr1 and the base-emitter junction of the second transistor Tr2 are connected in series to each other between the base terminal 1 and the emitter terminal 3. Therefore, reverse break down voltage across the base terminal 1 and the emitter terminal 3 of the composite Darlington transistor becomes twice that of a simple bipolar transistor. Therefore, in order to switch the Darlington transistor, a twice high reverse power than the simple transistor is necessary. Since the twice high reverse power is used, the fading-out of the reverse power requires a longer time than that for the simple bipolar transistor. In order to increase the switching speed of the transistor, it is necessary to make the Darlington transistor rapidly consume the reverse high power. In some examples of the present invention, a high speed of the switching is attained by providing the Darlington transistor with a means to efficiently consume the reverse high power.